Noxious substances such as CO, HC (hydrocarbons) and NO.sub.x are contained in exhaust gas, and catalysts have been used for cleaning such exhaust gas by changing said noxious substances into harmless materials such as CO.sub.2, H.sub.2 O and N.sub.2 through a redox reaction. The catalysts used for this purpose included two types: (1) those using noble metals such as Pt or Pd group metals and (2) those using oxides of transition metals such as Mn, Cu, Ce, etc. Noble metal catalysts are called three-way conversion catalyst and capable of simultaneously eliminating both reductive noxious gases such as CO and HC and oxidative noxious gas such as NO.sub.x. It is known, however, that these noble metal catalysts sinter and become aggregated, and lose their catalytic activity when they are exposed to a high temperature. On the other hand, oxide catalysts are used only for eliminating reductive noxious gases.
As to the sensors for detecting the stoichiometic ratio of combustion, there have been known the type in which platinum electrodes are set on both sides of the partition wall of stabilized zirconia solid electrolyte, one of said electrodes being exposed to an atmosphere where the partial pressure of oxygen is constant, such as air, while the other electrode being exposed to exhaust gas, thus forming an oxygen concentration cell, and a sudden change of its electromotive force is utilized for detecting the stoichiometric ratio, and the type in which a sudden change in electric resistance of a metal oxide such as SnO.sub.2, TiO.sub.2, MgCr.sub.2 O.sub.4, etc., is utilized. At an air/fuel ratio close to the stoichiometric ratio, the fuel is not perfectly burned and both reductive gases such as CO and HC and oxidative gases such as O.sub.2 and NO.sub.x coexist in the exhaust gas, so that there occurs no sudden change of electromotive force or electric resistance from the area of stoichiometric ratio of combustion unless said both types of gases are reacted with each other through the medium of a catalyst. In the systems utilizing the electromotive force of concentration cell, Pt of the electrode on the outside, which contacts with exhaust gas, performs said catalytic action, but in the systems utilizing the electric resistance of the metal oxide, almost no such catalytic action is occurred, so that it was necessary to add a noble metal to the metal oxide.
Recently, attempts have been made to use perovskite type metal oxides for exhaust gas cleaning catalysts or gas sensors, and the present inventors have also filed a patent application featuring the use of a material Sr.sub.(1+x)/2 La.sub.(1-x)/2 Co.sub.1-x Me.sub.x O.sub.3 (Me: Fe, Mn, Cr or V.delta.: loss of oxygen) (U.S. Pat. Nos. 4,314,996 and 4,485,191). This material exhibits a mixed conductivity of electrons and O.sup.2- ions, and the invention of this patent application clarified the relation of such mixed conductivity to the catalytic performance or sensor characteristics and provided a composition with an optimal ratio of components for obtaining the desired characteristics. It is known that generally the oxygen content in a metal oxide varies according to the partial pressure of oxygen in the gas atmosphere, resulting in a change of electric resistance of the metal oxide, and this principle is already applied in various gas sensors. Said material, however, has a quite characteristic property that it can stably maintain the same crystal structure regardless of any voluminous release or takeup of oxygen. In this material, O.sup.2- ionic migration (electric conduction) occurs through the deletion of oxygen ions in the crystal lattice, and under high temperature like in exhaust gas, such migration is faster than in other oxides. Therefore, this material shows a high catalytic cleaning performance under high temperature above 500.degree. C., and since the velocity of equilibrium reaction between reductive and oxidative gases is affected by such catalytic performance, it is possible to accelerate the responsiveness of the sensor for detecting stoichiometric composition with no need of using a noble metal catalyst. On the other hand, electric conduction of electrons occurs through conductive pairs of ##STR1## constituted by transition metal M (Co, Fe, Mn, Cr or V) in the B site of said compound oxide ABO.sub.3 and oxygen. When the temperature rises or the partial pressure of oxygen in the atmosphere lowers to cause release of oxygen, said conductive pairs disappear to increase electric resistance. To make the average valence number rate of transition metal 3.5 is to mamimize the number of said conductive pairs. The complex oxides of this type allow a greater reversible enlargement of oxygen loss than allowable for other oxides, although only to a point where .delta. is 0.5, but if the number of said conductive pairs is maximized, it becomes possible to so much widen the scope of change of resistance by release of oxygen, and this leads to a corresponding increase of sensitivity in use of said material for a sensor. To define the complex oxide composition to Sr.sub.(1+x)/2 La.sub.(1-x)/2 Co.sub.1-x Me.sub.x O.sub.3-.delta. was to maximize the deletion of oxygen ions and the number of electron conductive pairs. To substitute Co with one of Fe, Mn, Cr and V was to prevent .delta. from becoming greater than 0.5 even in a state of excess reductive gas. The complex oxides of such composition exhibit an excellent catalytic performance and sensor characteristics at high temperatures above 500.degree. C., but the ratio of ion conductivity to electron conductivity is still low level (10.sup.-4), and it has been required to further increase the ionic transference number for these uses. Also, the high thermal expansion coefficient, 20.times.10.sup.-6 /deg, of said material was a negative factor for combined use with other materials.
The prior art disclosures closest to the present invention are Lucas (U.S. Pat. No. 4,454,494) and Hitachi, Ltd. (U.S. Pat. No. 3,951,603), but these patents are different from the present invention in the following points: in Lucas patent, La is not the essential component, and also the second substance SrMeO.sub.3 (Me: Ti, Zr, Hf) is absent, while Hitachi patent has no SrMeO.sub.3.